Tunable Kondo effect in a bilayer graphene quantum channel

TL;DR

This study demonstrates tunable Kondo effects in bilayer graphene quantum point contacts, revealing transitions between SU(4) and SU(2) regimes driven by magnetic fields, with conductance features indicating strong many-body interactions.

Contribution

It provides experimental evidence of tunable SU(4) to SU(2) Kondo physics in bilayer graphene, highlighting the system's versatility for exploring many-body quantum phenomena.

Findings

Observation of Kondo temperatures from 0.5 to 2.4 K.

Detection of a transition from SU(4) to SU(2) Kondo regimes.

Kondo signatures persist under valley degeneracy breaking.

Abstract

The interaction between itinerant electrons and localized spins is key to a wide range of electronic phenomena. Of particular interest is the regime where the interacting electrons exhibit both spin and valley degeneracy, resulting in SU(4) Kondo physics. However, this regime is challenging to realize in typical mesoscopic systems because it requires a strong interaction between electrons, resulting in a Kondo temperature ($T_\mathrm{K}$) significantly larger than the spin and valley splittings. Here, we present conductance measurements of a quantum point contact (QPC) in bilayer graphene (BLG). Beyond the expected quantized conductance plateaus, which reflect spin and valley degeneracy, we observe an additional subband, known as `0.7 anomaly' exhibiting signatures of Kondo physics and a $T_\mathrm{K}$ ranging from approximately 0.5 up to 2.4 K at zero magnetic field, corresponding to Kondo energies between 40 and 200 $\mu$eV. Given that the spin-orbit splitting in BLG is between 40 and 80 $\mu$eV, we argue that these results are consistent with a transition between four-fold degenerate SU(4) and two-fold degenerate spin-valley locked SU(2) Kondo effects. Furthermore, we break the valley degeneracy of the lowest subband by an out-of-plane magnetic field and show that Kondo signatures remain present, indicating a transition from SU(4) to a valley-polarized SU(2) Kondo effect, and showing the versatility of BLG QPCs for exploring many-body effects.